http://www.kellymom.com/bf/pumping/milkcalc.html#calculator
<font color="ff0000">How much milk do babies need?</font>
Many mothers wonder how much expressed breastmilk they need to have available if they are away from baby.
In exclusively breastfed babies, milk intake increases quickly during the first few weeks of life, then stays about the same between one and six months (though it likely increases short term during growth spurts). Current breastfeeding research does not indicate that breastmilk intake changes with baby's age or weight between one and six months. After six months, breastmilk intake will continue at this same level until -- sometime after six months, depending in baby's intake from other foods -- baby's milk intake begins to decrease gradually (see below).
The research tells us that exclusively breastfed babies take in an average of 25 oz (750 mL) per day between the ages of 1 month and 6 months. Different babies take in different amounts of milk; a typical range of milk intakes is 19-30 oz per day (570-900 mL per day).
We can use this information to estimate the average amount of milk baby will need at a feeding:
* Estimate the number of times that baby nurses per day (24 hours).
* Then divide 25 oz by the number of nursings.
* This gives you a "ballpark" figure for the amount of expressed milk your exclusively breastfed baby will need at one feeding.
Example: If baby usually nurses around 8 times per day, you can guess that baby might need around 3 ounces per feeding when mom is away. (25/8=3.1).
<font color="ff0000">What if baby is eating solid foods?</font>
Sometime between six months and a year (as solids are introduced and slowly increased) baby's milk intake may begin to decrease, but breastmilk should provide the majority of baby's nutrition through the first year. Because of the great variability in the amount of solids that babies take during the second six months, the amount of milk will vary, too. One study found average breastmilk intake to be 30 oz per day (875 ml/day; 93% of total intake) at 7 months and 19 oz (550 ml/day; 50% of total energy intake) at 11-16 months.
Several studies have measured breastmilk intake for babies between 12 and 24 months and found typical amounts to be 14-19 oz per day (400-550 mL per day). Studies looking at breastmilk intake between 24 and 36 months have found typical amounts to be 10-12 oz per day (300-360 mL per day).
<font color="ff0000">Is baby drinking too much or too little expressed milk?</font>
Keep in mind that the amount of milk that baby takes at a particular feeding will vary, just as the amount of food and drink that an adult takes throughout the day will vary. Baby will probably not drink the same amount of milk at each feeding. Watch baby's cues instead of simply encouraging baby to finish the bottle.
If your baby is taking substantially more than the average amounts, consider the possibility that baby is being given too much milk while you are away. Things that can contribute to overfeeding include:
* Fast flow bottles. Always use the lowest flow bottle nipple that baby will tolerate.
* Using bottle feeding as the primary way to comfort baby. Some well-meaning caregivers feed baby the bottle every time he makes a sound. Use the calculator above to estimate the amount of milk that baby needs, and start with that amount. If baby still seems to be hungry, have your caregiver first check to see whether baby will settle with walking, rocking, holding, etc. before offering another ounce or two.
<font color="aa00aa">* Baby's need to suck. Babies have a very strong need to suck, and the need may be greater while mom is away (sucking is comforting to baby). A baby can control the flow of milk at the breast and will get minimal milk when he mainly needs to suck. When drinking from a bottle, baby gets a larger constant flow of milk as long as he is sucking. If baby is taking large amounts of expressed milk while you are away, you might consider encouraging baby to suck fingers or thumb, or consider using a pacifier for the times when mom is not available, to give baby something besides the bottle to satisfy his sucking needs.</font>
* If, after trying these suggestions, you're still having a hard time pumping enough milk, see I'm not pumping enough milk. What can I do?
If baby is taking significantly less expressed milk than the average, it could be that baby is reverse-cycling, where baby takes just enough milk to "take the edge off" his hunger, then waits for mom to return to get the bulk of his calories. Baby will typically nurse more often and/or longer than usual once mom returns. Some mothers encourage reverse cycling so they won't need to pump as much milk. Reverse cycling is common for breastfed babies, especially those just starting out with the bottle.
If your baby is reverse cycling, here are a few tips:
* Be patient. Try not to stress about it. Consider it a compliment - baby prefers you!
* Use small amounts of expressed milk per bottle so there is less waste.
* If you're worrying that baby can't go that long without more milk, keep in mind that some babies sleep through the night for 8 hours or so without mom needing to worry that baby is not eating during that time period. Keep an eye on wet diapers and weight gain to assure yourself that baby is getting enough milk.
* Ensure that baby has ample chance to nurse when you're together.
<font color="ff0000">Other ways of estimating milk intake</font>
There are various ways of estimating the amount of milk intake related to the weight of the baby and the age of the baby, based upon formula intake - research has shown that after the early weeks these methods overestimate the amount of milk that baby actually needs. These are the estimates that we used for breastfed babies for years, with the caveat that most breastfed babies don't take as much expressed milk as estimated by these methods. Current research tells us that breastmilk intake is quite constant after the first month and does not appreciably increase with age or weight, so the current findings are validating what moms and lactation counselors have observed all along.
So decided to get a extra manual pump from forum (just managed to deal with one of the WTS mummy). 
